Smelting is a well-established approach for recovering a metal, such as copper, from a metal-bearing sulfide material. Due to the high cost of smelting, however, the copper sulfide minerals in an ore body typically are first concentrated by flotation techniques to provide a smaller volume for smelting. The concentrate is then shipped to a smelter, which processes the concentrate pyrometallurgically at high temperatures to form a crude copper product that is subsequently refined to a highly pure metal.
The recovery of copper from copper sulfide concentrates using pressure leaching has proven to be a potentially economically attractive alternative to smelting. Pressure leaching operations generally produce less fugitive emissions than smelting operations, and thus, environmental benefits may be realized. Further, pressure leaching circuits may be more cost-effectively constructed on-site at a concentrator, eliminating the expense associated with concentrate transportation that smelting operations may require. Further, any byproduct acid produced in the pressure leaching circuit may be used in adjacent heap leaching operations, thus offsetting some of the costs associated with purchased acid.
The mechanism by which pressure leaching processes effectuate the release of copper from sulfide mineral matrices, such as chalcopyrite, is generally dependent on temperature, oxygen availability, and process chemistry. For example, in high temperature pressure leaching processes for chalcopyrite, that is, pressure leaching processes operating above about 200° C., it has generally been found that sulfur is fully converted to sulfate. In low temperature pressure leaching processes (i.e., below about 100° C.), it has generally been found that the chalcopyrite leaches slowly and incompletely. Medium temperature pressure leaching processes for chalcopyrite, which are generally thought of as those processes operating at temperatures from about 120° C. to about 190° C., have been the focus of much research and development in recent years and have shown some promise for achieving a satisfactory compromise between the high temperature and low temperature processes. As discussed in further detail hereinbelow, however, even with these efforts, such processes still exhibit significant processing disadvantages.
Low temperature pressure leaching processes historically have been disfavored because of characteristically low extraction of copper and other metals, and long residence times. High temperature pressure leaching processes, notwithstanding their relatively short residence times and high metal extractions, tend to have higher oxygen consumption, higher by-product acid production, and greater heat production in the pressure leaching vessel, which requires increased cooling. Prior medium temperature pressure leaching processes typically suffer incomplete copper extraction resulting from either passivation of the copper sulfide particle surfaces by a metal-polysulfide layer or partially-reacted copper sulfide particles becoming coated with liquid elemental sulfur and/or other reaction products. Further, in prior medium temperature processes, under certain conditions, molten elemental sulfur commonly agglomerates in the pressure leaching vessel to form coarse sulfur “prills” or “balls,” which inhibit the extraction of copper and other metals and which can create substantial difficulties with materials handling and transport.
A variety of previous attempts have been made to circumvent the problems associated with medium temperature pressure leaching and to realize the potential benefits pursuant thereto. For example, applying known pressure leaching processes to the treatment of zinc sulfide materials, previous attempts have been made to use surfactants such as lignin derivatives, tannin compounds (such as quebracho), and orthophenylene diamine (OPD) to disperse the elemental sulfur formed and to render the copper in chalcopyrite concentrates extractable. However, these attempts have not been entirely successful since relatively low copper extraction was realized even after significant residence times.
Other attempts have included pressure oxidation in the presence of an acidic halide solution (U.S. Pat. No. 5,874,055), and the use of finely divided particulate carbonaceous material to inhibit passivation of incompletely leached copper sulfide particles (U.S. Pat. No. 5,730,776). The feasibility of using molten sulfur-dispersing surfactants to enhance pressure leaching of chalcopyrite in the temperature range of 125° C. to 155° C. has been investigated; however, it was found that chalcopyrite particles (P90 of 25-38 microns) leached too slowly even if molten sulfur was prevented from passivating the material surfaces. See Hackl et al., “Effect of sulfur-dispersing surfactants on the oxidation pressure leaching of chalcopyrite,” proceedings of COPPER 95-COBRE 95 International Conference, Volume III, Electrorefining and Hydrometallurgy of Copper, The Metallurgical Society of CIM, Montreal, Canada. The authors of that study ultimately reported that the reaction rate for chalcopyrite was controlled, at least in part, by a passivating mechanism unrelated to sulfur formation.
It is generally known that hydrometallurgical processes, particularly pressure leaching processes, are sensitive to particle size. Thus, it is common practice in the area of extractive hydrometallurgy to finely divide, grind, and/or mill mineral species to reduce particle sizes prior to processing by pressure leaching. For example, U.S. Pat. No. 5,232,491 to Corrans, et al., entitled “Activation of a Mineral Species,” teaches a method of activating a mineral species for oxidative hydrometallurgy by milling the species to P80 of about 30 microns or less. Further, International Publication No. WO 01/00890 to Anglo American PLC, entitled “Process for the Extraction of Copper,” discusses pressure leaching of copper sulfide particles (P80 from 5-20 microns) in the presence of a surfactant material at temperatures from 130° C. to 160° C. According to test data set forth in this publication, pressure leaching of chalcopyrite under these conditions resulted in somewhat favorable copper extractions ranging from about 88.2 to about 97.9%.
It generally has been appreciated that reducing the particle size of a mineral species, such as, for example, copper sulfide, enables pressure leaching under less extreme conditions of pressure and temperature. The present inventors have observed, however, that in addition to being sensitive to the overall particle size distribution of the mineral species being processed, pressure leaching processes—namely, copper extraction by medium temperature pressure leaching processes—are sensitive to the coarsest particle sizes in the process stream above about 25 microns. Indeed, photomicrographs of autoclave residue from coarse-ground (i.e., P80 of about 30-100 microns) chalcopyrite feed material have indicated that unreacted chalcopyrite particles coarser than about 20 microns were encapsulated in elemental sulfur. It was observed that very few chalcopyrite particles finer than about 10 microns remained in the residue.
An effective and efficient method to recover copper from copper-containing materials, especially copper from copper sulfides such as chalcopyrite and chalcocite, that enables high copper recovery ratios at a reduced cost over conventional processing techniques would be advantageous.